Apple is reportedly clamping down on apps downloaded through its App Store.

The company has told some app developers that they can no longer sell content within their apps or give app users access to content purchased outside the Apple App Store, according to a New York Times report.

Apple recently rejected an iPhone app that would have allowed customers to purchase and read e-books sold through the Sony Reader Store, and Apple told Sony that all in-app purchases would now go through Apple, Steve Haber, president of Sony's digital reading division, told the newspaper.

Apple representatives did not immediately respond to a request for comment.

The new policy would effectively cut off Amazon.com and other companies that sell e-readers that compete with Apple's iPad, the Times noted. Many e-readers offer mobile apps that allow content purchased in their online stores to be read on other devices, including the iPad. Last year, Amazon released a free Kindle app for iPad and other tablets, allowing users of tablets other than Amazon's Kindle access to more than 450,000 e-books.

Apple--already the biggest music retailer in the world--may now be turning its eye toward conquering the e-book market, which Amazon currently dominates. Apple is rumored to be looking to expand its market for digital media consumption on its tablet with a news venture with News Corp. featuring content exclusively for the iPad. News Corp. CEO Rupert Murdoch is expected to announce the partnership with Apple tomorrow in New York.

A tightening hold over the App Store would also appear to upset a recent detente Apple has created with app developers, who were often left frustrated and in the dark when an app was rejected by the App Store. But Apple recently announced plans to publish its App Store Review Guidelines and to allow developers to create applications with just about any tool they want.